Brake problems - Contaminated Rotors ??

SLX brakes which up until recently performed flawlessly. However the front is causeing problems.

At the start of a ride, there is little power even with the lever pulled as hard as I can and the squeal is horrific. It gets gradually better into the ride as I am using them, however, once the ride is finished and bike packed away, its back to the same problem on the next ride. I've changed the pads which initially helped, before it starting again. They are generally ok after a lot of use.

Could it be the rotor ? If so can you recommend a good way of cleaning them ?

Shimano calipers do have a habit of leaking a tiny amount of fluid from the piston seal
I could never see my xt one leaking but after having several set on pads fail I changed the caliper and no more problems

when I suffered the weeping seal it wasn't untill the calipers started to heat up that power was lost .The discs usually self clean due to heat.It may be air in the line try taking pads out and heating them up over a gas flame ,if they are contaminated they will smell of burning oil.check that both pistons are equally out and fluid level in resevoir is ok ,bleed if old oil, and or tie the lever back to the bar to try and purge the air out of the line

Problem is, you have to ditch them anyway as the calipers can't have their seals replaced. The calipers are spraying a very fine mist of brake fluid onto the rotors / pads. If you replace the pads, it will work for a while, but soon the squeal and crap performance will come back.